3 things we learned from Barcelona's battering of error-strewn City

Lionel Messi laid waste to Manchester City's slips - both literally and figuratively - as Barcelona trounced the latest students of Camp Nou favourite Pep Guardiola in Wednesday's Champions League group-stage clash.

While John Stones was schooled when his lack of awareness and experience was pounced upon by Messi for his hat trick, the diminutive world beater was the beneficiary of Fernandinho losing his footing for his opener, and then City being in disarray after Claudio Bravo's dismissal to nick his brace.

Jeremy Mathieu, a first-half replacement for the injured Gerard Pique, also received his marching orders for two yellow cards, but City was spluttering and had nothing in the tank to prevent Neymar dancing through late on for Barca's fourth.

Here are three things we learned from Manchester City getting its pants pulled down on the Mediterranean coast:

Bravo blooper - again

Just eight matches into his City career, Bravo is subject to intense scrutiny; and the prominent queries are over what he can do that Joe Hart can't.

Preferred ahead of the loaned-out crowd favourite, Bravo's tough start reached new depths at his old club when his egregious give-away to Luis Suarez culminated in the Chilean flapping at the Uruguayan's lob some 5 yards outside his area.

Related - Watch: Bravo sent packing after ridiculous error vs. Barcelona

The questions levelled at Bravo - 33 years old and an €18-million-plus transfer fee - are legitimate. His flap on his debut in a Manchester derby has been followed by some decent saves and attack-instigating passing from deep, but he's old, expensive, and his dwelling on the ball has been targeted as a City soft spot by numerous opponents.

City has an option to bring Geronimo Rulli from Real Sociedad permanently in any of the next three windows. Guardiola must be tempted.

Defensive concerns for Barca

Barcelona may have to patch up some defensive holes over the next week or so.

Both Jordi Alba and Pique had to be substituted with issues in the opening stanza and, although the subsequent swapping and changing - Samuel Umtiti cropping up on the right at one point, Lucas Digne filling in on the left for a while - wasn't ideal, the defence went relatively untroubled.

Mathieu's red card only extends the worries that Enrique will have, however. A trip to a steadily improving Valencia is next up and, in two weeks' time, the reverse fixture with City in Group C, for which Mathieu will be suspended. That should prove a much sterner test.

The greatest aspect of Barcelona's win was its ruthlessness rather than its defending, though. Messi and Neymar were obvious candidates to guide their team to victory, but making mistakes against the Blaugrana - something City threatened to do early in the match through misplaced passes from David Silva and Stones - is going to be capitalised upon.

Barcelona has scored 13 goals in its first three matches of this Champions League campaign.

No Aguero-shaped hole evident

It seems obscene for a player with 28 goals in his last 29 Premier League outings, but maybe Guardiola wants more from Sergio Aguero.

Related: Aguero benched for Barcelona trip in Guardiola's latest reshuffle

The overlooked Aguero's fine five-plus years at Manchester City have seen the 28-year-old stubbornly occupying the most advanced position in the lineup, often displaying great frugality to his movement as he turns away from a marker to steal half a yard. He doesn't tend to lace it, either; rather nudging the ball into the corner, beyond the reach of a despairing goalkeeper.

The latter aspect of the former Atletico Madrid man's game certainly won't displease his gaffer, but his work in tight areas doesn't exactly match Guardiola's designs. The tactician borrows his ideals from the Rinus Michel and Johan Cruyff-championed Total Football philosophy, with a fluidity of movement and no nailed-down positions for those fielded higher than the backline (or, in City's case, Fernandinho). Aguero's expected to contribute to more phases of play - Something that Suarez, a player of similar attributes to Aguero, has done superbly at Barcelona.

"Luis is one of the best strikers in modern football so he's a player who for every defender it is difficult to play against," right-back and Wednesday's skipper Pablo Zabaleta told the pre-match press conference.

"He's very intelligent, with ball ability to create chances and define, without the ball always closing down spaces and trying to start the play and go out to the sides. It's important not to give him any advantage. If we give him a chance it could be difficult for us."

Aguero's benching ultimately worked as City went into the break undeservedly down 1-0. It was an unfortunate Fernandinho slip and Bravo gaffe that were City's undoing in the 4-0 defeat, and not the selection policies of Guardiola.